1. Field of the Invention
The present invention relates to particles for an electro-rheological fluid, more specifically, particles for an electro-rheological fluid, comprising spherical carbonaceous particles of high strength.
2. Description of the Related Art
Electro-rheological fluids significantly and reversibly change their rheological characteristics under electrical control. The winslow effect, which is the phenomenon of dramatic change of the apparent viscosity of a fluid through the application of an electric field, has been known for a long time. The application of this effect for electrically controlling devices or parts, such as clutches, valves, engine mounts, actuators, and robot arms has been discussed. However, electro-rheological fluids in the early days were ones comprising particles such as starch dispersed in mineral oil or a lubricant, with the drawback of poor reproductivity. The electro-rheological effect, however, can be provided.
Many proposal have been made mainly on particles used as a dispersoid aiming at obtaining a fluid having a high electro-rheological effect and excellent reproductivity. For example, Japanese Patent Application Laid-Open (JP-A) No. 53-93186 discloses a highly water-absorbent resin having an acidic group such as polyacrylic acid. Japanese Patent Application Publication (JP-B) No. 60-31211 discloses an ion exchange resin, and JP-A No. 62-95397 discloses alumina silicate. These are hydrophilic solid particles. They are soaked in water and dispersed in an insulating oil-like medium. It is said that polarization generates in the particles comprising the particles through the action of water upon applying a high voltage from the outside, and subsequently the viscosity increases owing to the crosslinking among the particles in the electric field direction by the polarization.
However, the above-mentioned hydrous type electro-rheological fluids using hydrous particles have many problems such as incapability of having a sufficient electro-rheological effect over a wide temperature range, limitations of temperature in usage to avoid evaporation or freezing, increase in electric current consumption with temperature rises, instability caused by water transfer, and corrosion of electrode metals at the time of high voltage application, and thus it has been difficult to make practical use of them.
In order to solve these problems, anhydrous electro-rheological fluids including water-free particles have been proposed. For example, JP-A No. 61-216202 discloses organic semiconductor particles such as polyacene quinone, JP-A Nos. 63-97694 and 1-164823 disclose thin film-coated type composite particles including essentially dielectric particles prepared by forming a conductive thin film on the surface of organic or inorganic solid particles, and further forming an insulating thin film thereon, that is, a thin film having conductive/insulating electric characteristics. Furthermore, as for dispersoid particles, surface-treated metal particles and metal-covered inorganic particles are known. However, an anhydrous electro-rheological fluid has not been provided for a practical use due to various problems such as lack of electro-rheological effect with low electric power consumption, difficulty in industrial production, and availability only in an alternating current electric field.
In order to further improve electro-rheological effects in anhydrous electro-rheological fluids with low electric power consumption, it is necessary to increase the filling ratio of the dispersoid powders. However, this causes the initial viscosity of the fluid to increase and consequently the electro-rheological effect at the time of electric current application is reduced.
As a method of solving the problem JP-A No. 7-90287 discloses an electro-rheological fluid using spherical carbonaceous particles. It is advantageous to use homogeneous spherical carbonaceous particles as particles of the electro-rheological fluid, however, when the electro-rheological fluid is applied to an engine mount, an actuator, or a clutch, the particles are destroyed by the strain of vibration or shearing causing an increase in the viscosity when no electric field is applied. This and insufficient durability due to particle strength are problems.
The present invention involves improvement of the durability of the particles for electro-rheological fluid as well as further improvement of the electro-rheological effect.
An object of the present invention is to provide particles for electro-rheological fluid providing a high electro-rheological effect over a wide temperature range with low electric power consumption, and having high strength and excellent durability, and not being susceptible to break-up under stress.
The particles for electro-rheological fluid of the present invention are spherical carbonaceous particles, substantially obtained from a solvent and a condensation product of a methylene type bond of aromatic sulfonic acid or a salt thereof.
In particles for an electro-rheological fluid of the present invention, the above-mentioned spherical shape has a deviation of the minimum diameter of the carbonaceous particles within 30% of the average diameter.
Furthermore, it is preferable that particles of electro-rheological fluid, including spherical carbonaceous particles, of the present invention have physical properties such as a collapsing strength of 5 kgf/mm2 or more, a maximum displacement amount of 3% or more, an ash content of 0.1% or less, and an average particle size of 0.1 to 20 xcexcm.
Although it is preferable to have low initial viscosity to enhance electro-rheological effects in an electro-rheological fluid, in conventional particles for electro-rheological fluid, with a high filling ratio of particles, the initial viscosity increases accordingly, and consequently it is difficult to obtain high electro-rheological effects. However, since electro-rheological fluid utilizing spherical carbonaceous particles obtained from specific materials of the present invention have spherical particles, do not cause a drastic rise in viscosity despite an increased filling ratio, and have high strength without much risk of break-up under stress, excellent durability and effective electro-rheological effects can be obtained. Unlike amorphous fine particles, increases in electric current consumption due to local voltage rises derived from unevenness of the particle density are believed not to occur.
Particles of the electro-rheological fluid of the present invention when acting as an electro-rheological fluid have low initial viscosity and high electro-rheological effects at low power consumption over a wide temperature range. Furthermore, even when in use at a high shear rate in a device over a long period of time, since the particles have a large resistance to the shearing force and high strength to break-up, an excellent durability is shown without break-up of the particles or any increase in fluid viscosity when no electric field is applied.